Storage will be all solid state at some point in the future, but the vision of an all-flash data centre is still some way off, according to Nimble Storage.

Disk is still significantly cheaper than flash, according to Nimble Storage

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Storage will be all solid state at some point in the future, but the vision of an all-flash data centre is still some way off, according to Nimble Storage.

There are two arguments that vendors use for the flash-only array. One is that the price of flash and disk is rapidly converging, and the other is that flash-only arrays provide more consistent performance than hybrid arrays that combine flash and disk.

According to Varun Mehta, co-founder and VP of Engineering at Nimble Storage, however, both of these claims need "very careful examination".

Nimble Storage believes that flash is still ten times more expensive than disk on a per-Gigabyte basis - and this is for low-end flash storage. Dave Goulden, President and COO of EMC, recently quoted a 40x price differential.

"The disk guys are talking about technologies such as shingled writes, heat-assisted magnetic recording (HAMR), and helium-filled drives that extend the capacity of disks for a long, long time. So I think flash will have to be very adept at trying to keep up with disk," said Mehta.

Meanwhile, the argument that you need flash-only arrays for consistent performance is made by people who "just don't understand computer architecture very well," according to Mehta.

"If you look at how a computer is built up, it's a series of caches - there are caches upon caches, and that's just the hardware. The operating system and the applications have caches too, so to say that an architecture that relies on caching is somehow inefficient is arguing against all of computer science in a way."

Flash is a cornerstone of Nimble's product portfolio, but co-founder and CTO Umesh Maheshwari explained that flash has limitations on how many times you can write to it. After you have written to it a few thousand times it wears out, so you can lose your data.

"There are ways to mitigate these problems, but they require even more resources, so you need more redundancy, maybe higher-end flash - that's even more costly, and even after that some of the questions remain," he said.

It is for this reason that Nimble advocates combining disk and flash in a way that users get both the performance benefits of flash and the capacity advantages of disk.

The most common approach is to put the "hot" data in flash and the "cold" data on disk, but the problem is that the hot data often shifts. So in the case of Microsoft Exchange, for example, the "hot" data is the previous weeks emails - and these change on a weekly basis.

If data becomes hot, the user may want to promote it from disk to flash, so some data will have to be demoted from flash back to disk. This requires disk I/O, which is very expensive. The system therefore needs to work out whether data is really hot before it promotes it, and that can take hours or even days.

"If the system not responsive to a changing workload, then people will end up putting all of their Exchange in flash, and that defeats the purpose of having a hybrid system in the first place," said Maheshwari.

Nimble therefore treats flash not as an endpoint of storage but as an accelerator on top of the disk, so all the data can reside on the disk, and the flash effectively acts as a cache. This reduces the load on the disk subsystem, because there are fewer residual reads going to the back end.

The company is now one of the few storage vendors certified to sell its storage arrays pre-packaged with Cisco's UCS, alongside EMC and NetApp. It hopes that this partnership will help it to extend its customer base from the mid-market to the enterprise.

"What is clear is that disk as a mechanical medium at some point will go away, but I don't think we will be there for a few years to come - at least in my mind, it will be somewhere between 5 and 15 years from now," said Maheshwari.

"In our mission, flash does not appear anywhere. It says to build the most efficient storage systems. So today a combination of flash and disk is the best to get there."